Remove muffins from pan after baking using a knife or metal spatula instead of inverting and losing streusel.
1. Preheat oven to 375°. Stir together chopped peanuts, peanut flour, 1/2 cup all-purpose flour, 1/4 cup light brown sugar, and 3 1/2 tablespoons melted butter until mixture is crumbly.
2. Stir together baking soda, next 3 ingredients, and remaining 1 1/2 cups all-purpose flour in a bowl.
3. Process mango in a blender or food processor 15 to 20 seconds or until pureed. Pour puree into a measuring cup to equal 1 cup. (If necessary, discard any remaining puree, or reserve for another use.)
4. Whisk together 1 cup mango puree, granulated sugar, buttermilk, egg, and remaining 1/4 cup brown sugar and 2 tablespoons melted butter in a medium bowl until well blended. Gradually whisk in flour mixture just until combined.
5. Place baking cups in a muffin pan. Spoon batter into cups, filling two-thirds full. Sprinkle peanut streusel mixture over batter.
6. Bake at 375° for 22 to 24 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 minutes. Gently transfer muffins from pan to a wire rack using a knife or metal spatula, and cool completely (about 20 minutes).
*2 cups frozen cubed mango, thawed, may be substituted.
Note: To make ahead, freeze baked, cooled muffins in a heavy-duty zip-top plastic freezer bag up to 1 month. To reheat, remove muffins from baking cups, wrap in a paper towel, and microwave at HIGH 20 seconds or until thoroughly heated. For testing purposes only, we used 12% fat light roast peanut flour.